Engine oils function under severe oxidative conditions. The oxidative breakdown of the engine oil creates sludge and deposits, deteriorates the viscosity characteristics of the oil, and produces acidic bodies that corrode engine parts. To combat the effects of oxidation, engine oils are formulated with an array of antioxidants including hindered phenols, aromatic amines, zinc dithiophosphates (ZDDP), sulfurized hydrocarbons, metal and ashless dithiocarbamates, and organo-molybdenum compounds. Particularly effective antioxidants are alkylated diphenylamines (ADPAs), and ZDDPs. In combination, these two compounds provide the majority the of the antioxidant capacity in engine oils under current practice. In addition, ZDDP is the main source of antiwear protection for engine oils. However, the use of ZDDP in engine oils is declining due to the poisoning effect of phosphorus on exhaust after-treatment catalyst. In addition, sulfur levels in engine oils are also in decline due to the effect of sulfated ash exhaust after-treatments. Thus, a need exists for effective antioxidant chemistry that can reduce or eliminate the need for phosphorus and sulfur containing antioxidants and antiwear additives.
In U.S. Patent Application 2004/0214731 A1, Tynik discloses that organoammonium tungstate compounds are effective antiwear additives without contributing phosphorus or sulfur to a lubricating composition The invention herein teaches that unlike ZDDP, these organoammonium tungstate compounds alone do not effectively inhibit oxidation of lubricating compositions. However, in the presence of secondary diarylamines, organoammonium tungstate compounds acts synergistically to provide oxidation control much improved over either of the components separately. Thus, organoammonium tungstates represent a technology that will reduce or eliminate the need for phosphorus and sulfur containing additives such as ZDDP.